<p>Conventional heatwave assessments are often based on individual metrics that cannot fully capture the combined characteristics and nonlinear variability of heatwave activity. Using daily maximum temperature data from the ERA5 reanalysis over China during 1940–2024, we apply a Heatwave Proneness Index (HWPI) together with the Ensemble Empirical Mode Decomposition (EEMD) method to investigate the spatial patterns, temporal variability and nonlinear trends of heatwaves. The results reveal pronounced spatial heterogeneity in heatwave characteristics, with generally higher values over Southwest China, the Qinghai–Tibet Plateau, and a southwest-northeast belt, while Northwest China is characterized by a low-frequency but high-intensity heatwave regime. Significant increasing linear trends in HWPI are detected across most of China, consistent with the climatological mean pattern. Beyond these spatial and linear trend features, EEMD analysis further reveals strong temporal variability in heatwave trends, characterized by changes in trend rates and transition behavior across China. In recent decades, the rate of nonlinear change in heatwaves has accelerated markedly in western China. Notably, despite an insignificant linear trend in Xinjiang, the nonlinear trend component shows a clear recent intensification. Two main nonlinear patterns are observed in China, persistent increase and early weakening followed by intensification, showing spatially staggered transitions from northern regions in the 1950s to inland areas by the 1970s, indicating that the timing of heatwave intensification varies across regions rather than occurring simultaneously nationwide. These findings highlight the importance of accounting for nonlinear changes when assessing the long-term changes of heatwaves under ongoing climate warming.</p>